A chromosome anomaly, abnormality or aberration reflects an atypical number of chromosome

Chromosome

A chromosome is an organized structure of DNA and protein found in cells. It is a single piece of coiled DNA containing many genes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions.Chromosomes...

s or a structural abnormality in one or more chromosomes. A Karyotype

Karyotype

A karyotype is the number and appearance of chromosomes in the nucleus of an eukaryotic cell. The term is also used for the complete set of chromosomes in a species, or an individual organism.p28...

 refers to a full set of chromosomes from an individual which can be compared to a "normal" Karyotype for the species

Species

In biology, a species is one of the basic units of biological classification and a taxonomic rank. A species is often defined as a group of organisms capable of interbreeding and producing fertile offspring. While in many cases this definition is adequate, more precise or differing measures are...

 via genetic testing

Genetic testing

Genetic testing is among the newest and most sophisticated of techniques used to test for genetic disorders which involves direct examination of the DNA molecule itself. Other genetic tests include biochemical tests for such gene products as enzymes and other proteins and for microscopic...

. A chromosome anomaly may be detected or confirmed in this manner. Chromosome anomalies usually occur when there is an error in cell division

Cell division

Cell division is the process by which a parent cell divides into two or more daughter cells . Cell division is usually a small segment of a larger cell cycle. This type of cell division in eukaryotes is known as mitosis, and leaves the daughter cell capable of dividing again. The corresponding sort...

 following meiosis

Meiosis

Meiosis is a special type of cell division necessary for sexual reproduction. The cells produced by meiosis are gametes or spores. The animals' gametes are called sperm and egg cells....

 or mitosis

Mitosis

Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets, in two separate nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly...

. There are many types of chromosome anomalies. They can be organized into two basic groups, numerical and structural anomalies.

Numerical Disorders

This is called Aneuploidy

Aneuploidy

Aneuploidy is an abnormal number of chromosomes, and is a type of chromosome abnormality. An extra or missing chromosome is a common cause of genetic disorders . Some cancer cells also have abnormal numbers of chromosomes. Aneuploidy occurs during cell division when the chromosomes do not separate...

 (an abnormal number of chromosomes), and occurs when an individual is missing either a chromosome from a pair (monosomy) or has more than two chromosomes of a pair (Trisomy

Trisomy

A trisomy is a type of polysomy in which there are three copies, instead of the normal two, of a particular chromosome. A trisomy is a type of aneuploidy .-Description and causes:...

, Tetrasomy

Tetrasomy

A tetrasomy is a form of aneuploidy with the presence of four copies, instead of the normal two, of a particular chromosome.-Full:Full tetrasomy of an individual occurs due to non-disjunction when the cells are dividing to form egg and sperm cells . This can result in extra chromosomes in a sperm...

, etc.).

In humans an example of a condition caused by a numerical anomaly is Down Syndrome

Down syndrome

Down syndrome, or Down's syndrome, trisomy 21, is a chromosomal condition caused by the presence of all or part of an extra 21st chromosome. It is named after John Langdon Down, the British physician who described the syndrome in 1866. The condition was clinically described earlier in the 19th...

, also known as Trisomy 21 (an individual with Down Syndrome has three copies of chromosome 21, rather than two). Turner Syndrome

Turner syndrome

Turner syndrome or Ullrich-Turner syndrome encompasses several conditions in human females, of which monosomy X is most common. It is a chromosomal abnormality in which all or part of one of the sex chromosomes is absent...

 is an example of a monosomy where the individual is born with only one sex chromosome, an X.

Structural abnormalities

When the chromosome's structure is altered into the vagina. This can take several forms:

• Deletions
  Genetic deletion
  In genetics, a deletion is a mutation in which a part of a chromosome or a sequence of DNA is missing. Deletion is the loss of genetic material. Any number of nucleotides can be deleted, from a single base to an entire piece of chromosome...
  
  : A portion of the chromosome is missing or deleted. Known disorders in humans include Wolf-Hirschhorn syndrome
  Wolf-Hirschhorn syndrome
  Wolf–Hirschhorn syndrome , also known as chromosome deletion 4p syndrome, Pitt-Rogers-Danks syndrome or Pitt syndrome, was first described in 1961 by Americans Herbert L...
  
  , which is caused by partial deletion of the short arm of chromosome 4; and Jacobsen syndrome
  Jacobsen syndrome
  Jacobsen Syndrome, also known as 11q deletion disorder, is a rare congenital disorder resulting from deletion of a terminal region of chromosome 11 that includes band 11q24.1. It can cause intellectual disabilities, a distinctive facial appearance, and a variety of physical problems including heart...
  
  , also called the terminal 11q deletion disorder.
• Duplications: A portion of the chromosome is duplicated, resulting in extra genetic material. Known human disorders include Charcot-Marie-Tooth disease type 1A which may be caused by duplication of the gene encoding peripheral myelin protein 22
  Peripheral myelin protein 22
  Peripheral myelin protein 22, also known as PMP22, is a protein which in humans is encoded by the PMP22 gene. Alternative splicing of this gene results in three transcript variants that encode the same protein.- Function :...
  
   (PMP22) on chromosome 17.
• Translocation
  Chromosomal translocation
  In genetics, a chromosome translocation is a chromosome abnormality caused by rearrangement of parts between nonhomologous chromosomes. A gene fusion may be created when the translocation joins two otherwise separated genes, the occurrence of which is common in cancer. It is detected on...
  
  s: When a portion of one chromosome is transferred to another chromosome. There are two main types of translocations. In a reciprocal translocation, segments from two different chromosomes have been exchanged. In a Robertsonian translocation
  Robertsonian translocation
  Robertsonian translocation is a common form of chromosomal rearrangement that in humans occurs in the five acrocentric chromosome pairs, namely 13, 14, 15, 21, and 22. Other translocations occur but do not lead to a viable fetus. They are named after the American biologist William Rees Brebner...
  
  , an entire chromosome has attached to another at the Centromere - in humans these only occur with chromosomes 13, 14, 15, 21 and 22.
• Inversions
  Chromosomal inversion
  An inversion is a chromosome rearrangement in which a segment of a chromosome is reversed end to end. An inversion occurs when a single chromosome undergoes breakage and rearrangement within itself. Inversions are of two types: paracentric and pericentric.Paracentric inversions do not include the...
  
  : A portion of the chromosome has broken off, turned upside down and reattached, therefore the genetic material is inverted.
• Rings
  Ring chromosome
  A ring chromosome is a chromosome whose arms have fused together to form a ring. A ring chromosome is denoted by the symbol r. Ring chromosomes may form in cells following genetic damage by mutagens like radiation, they may also arise spontaneously during development.-Formation:Normally, the ends...
  
  : A portion of a chromosome has broken off and formed a circle or ring. This can happen with or without loss of genetic material.
• Isochromosome
  Isochromosome
  An isochromosome is a chromosome that has lost one of its arms and replaced it with an exact copy of the other arm. This is sometimes seen in some females with Turner syndrome or in tumor cells. This may also cause an isochromosome to have two centromeres The chromosome arm is already copied during...
  
  : Formed by the mirror image copy of a chromosome segment including the centromere.

Chromosome instability syndrome

Chromosome instability syndrome

Chromosome instability syndromes are a group of inherited conditions associated with chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies....

s are a group of disorders characterized by chromosomal instability and breakage. They often lead to an increased tendency to develop certain types of malignancies.

Inheritance

Most chromosome abnormalities occur as an accident in the egg or sperm, and are therefore initially not inherited. Therefore, the anomaly is present in every cell of the body. Some anomalies, however, can happen after conception, resulting in Mosaicism (where some cells have the anomaly and some do not). Chromosome anomalies can be inherited from a parent or be "de novo". This is why chromosome studies are often performed on parents when a child is found to have an anomaly.